Systems are known in which customers of a centralized utility such as a power plant, for example, receive switching signals designed to modify the operation of a rate meter at certain times, e.g. in the evening for a changeover to a lower night rate and in the morning for a return to the normal day rate. In the case of an electric-power station these signals generally appear as low-amplitude modulations, at a predetermined elevated frequency, of the utility current delivered to the customer at 50 or 60 Hz. Since the waveform of that utility current is rarely a pure sine curve, a highly selective filter is needed to separate such a signal from adjacent harmonics of the basic frequency.
Passive filters, as a class, do not have the necessary selectivity and thermal stability to satisfy the requirements of such a signaling system under diverse ambient conditions. An improvement in selectivity can be realized with the use of purely resistive/capacitive active circuits comprising a pair of relatively detuned filter sections whose coupling factor k is given by ##EQU1## WHEREIN .omega..sub.1 AND .omega..sub.2 ARE THE NATURAL PULSATANCES OF THE TWO COUPLED SECTIONS. The amplification factor .alpha. of such a filter is a function of its coupling factor k, as is the damping factor d. With filter impedances consisting for example of metal-layer resistors and capacitors of the type marketed under the trademark Styroflex, the resonance frequencies of the individual filter sections are found to vary by as much as .+-.2.05% in response to temperature variations between -30.degree. and +70.degree. C. With a damping factor d = 2%, a value suitable for signaling systems of the type discussed above, a coupling factor k nominally equal to d will vary between -2.1% and +6.1%. The amplification factor .alpha. fluctuates in that instance between 200% (k = 0) and 19.4% (k = 6.1%) which, of course, would be intolerable in such a switching system.